# Theory Seminars WS 20/21

### Thu 29.10.2020, 16.30 h

Coenraad Marinissen (NIKHEF and Utrecht University)

Efficient Hilbert Series for Effective Theories

**Abstract:**

The task of finding operator bases for effective theories can be assisted by using the Hilbert series, which counts the number of independent operators at a given effective order. In this talk I will introduce the Hilbert series for Standard Model Effective Theory and some extensions, and present ECO, a FORM based implementation that can be used to efficiently establish the number of operators at effective orders as high as 20 (or more). I will discuss some applications, and give an outlook on interesting extensions to other effective theories.

Host: R. Harlander

### Thu 05.11.2020, 16.30 h

Karsten Jedamzik (Montpellier U.)

On the Hubble tension and primordial magnetic fields

Abstract:

My talk will have three parts. First, I introduce the audience to the gross features of primordial magnetic field evolution before recombination. Then, I will show how primordial magnetic fields can help relieve the Hubble tension. Finally, I make some general statements about a large number of beyond LambdaCDM models proposed to solve the Hubble tension and why they usually do not completeley achieve this goal.

Host: J. Lesgourgues

### Thu 12.11.2020, 16.30 h

Valentin Hirschi (ETH Zürich)

Local unitarity: a new paradigm for perturbative computations in QFT

**Abstract:**

The canonical approach for tackling higher-order computations involves splitting up a finite quantity into two separately divergent ones. First, multiloop amplitudes are evaluated using (mostly) analytical techniques. Second, phase-space integrals are computed using numerical Monte-Carlo techniques with ad-hoc subtraction terms for regularising soft and/or collinear infrared divergences. We will present our recent work, Local Unitarity, in which we propose a radically different technique whereby the singularities in real-emission and virtual degrees of freedom directly cancel without the use of any counterterms and at any perturbative order. On the practical side, this allows for the fully numerical computation of differential corrections, potentially alleviating the complexity bottleneck analytical alternatives are facing. From a theoretical standpoint, our proof of the local cancellation of infrared singularities sheds new light on the exact mechanism by which it is realised.

Host: M. Czakon

### Thu 19.11.2020, 16.30 h

Ilias Cholis (Oakland University)

Scrutinizing Gamma-Ray Observations from Milky Way's Center in Search of Dark Matter Signals

**Abstract:**

The nature of dark matter is one of the most puzzling questions in physics. We observe its gravitational impact in many different astronomical systems and through cosmological measurements have now determined with great precision its abundance in the Universe. I will discuss how we search for possible signals of dark matter using observations at gamma-rays and how we address the challenges of both the astrophysical uncertainties and alternative astrophysical sources that can give signals mimicking those from dark matter. Using a recently released point source catalogue by the Fermi Large Area Telescope Collaboration and performing a template fit with my collaborators we have confirmed the presence of the previously claimed diffuse emission component known as the ``Galactic Center Excess'' in gamma-ray observations at GeV energies. Moreover, through a wavelet-based search we have placed constraints on the properties of point sources that could possibly explain that diffuse emission seen in the template fit.

Host: P. Mertsch

### Thu 26.11.2020, 16.30 h

Mathieu Pellen (Universität Freiburg)

Full NLO QCD corrections to off-shell ttbb production

**Abstract:**

The physics program of the LHC offers great opportunities to probe the fundamental structure of the Standard Model. To that end, precise theoretical predictions for both signal and background are required. A primary example of it is the measurement of the ttH signal which possesses the ttbb process as irreducible background. The physics and the challenges associated to it will be discussed in this talk. In particular I will present a new computation of the full NLO QCD corrections to off-shell ttbb production.

Host: M. Worek

### Thu 03.12.2020, 16.30 h

Mauro Pieroni (Imperial College London)

Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA

**Abstract:**

In this seminar I will discuss Stochastic Gravitational Wave Backgrounds (SGWB) characterization with LISA. After a general introduction on SGWB detection, I will explain the peculiar features of LISA with a focus on the response function and on the noise spectrum. The core of my talk will be the presentation of two different methods for model independent SGWB frequency reconstruction:

- The so-called ``binning method'' (1906.09244, 2009.11845) based on the idea of approximating the signal with a piecewise-defined function where each sub-function is a power law.

- A more recent method (2004.01135) based on principal components analysis. The problem of component separation to disentangle a cosmological SGWB from astrophysical foregrounds is addressed in both cases.

Host: J. Lesgourgues

### Thu 10.12.2020, 16.30 h

Marco Bonvini (INFN, Rome)

Probabilistic definition of the perturbative theoretical uncertainty from missing higher orders

**Abstract:**

In many branches of physics, and in particular in quantum field theory, the perturbative approach offers a powerful tool for making theoretical predictions. When the expansion parameter is not small enough, as it happens in quantum chromodynamics, and the number of computable orders is limited, it becomes very important to be able to estimate the uncertainty on a theoretical prediction due to the missing higher orders. This is particularly important when comparing theory with data, e.g. for the precision physics programme of the LHC. I will review the standard method, which is based on unphysical scale variation. While scale variation is certainly a good tool to guess the size of the next perturbative order, it lacks of a probabilistic interpretation and it often underestimates the actual uncertainty. A few years ago Cacciari and Houdeau proposed a Bayesian model to give a statistical meaning to theory uncertainties. I will review the Cacciari-Houdeau approach, and present new models which improve significantly the accuracy of the original approach. I will further show how scale dependence can be "removed" from a perturbative result within the context of these models.

Host: M. Worek

### Thu 17.12.2020, 16.30 h

Tracy Slatyer (MIT)

Deciphering the Universe's Dark History

**Abstract: **

Dark matter constitutes more than 5/6 of the matter in the universe, but its nature and interactions remain one of the great puzzles of fundamental physics. Dark matter collisions or decays have the potential to produce high-energy particles; such particles may already have reshaped the history of our cosmos, leaving traces of their existence in ionization and heating of the intergalactic medium, and in background radiation from the cosmic dark ages and the epoch of reionization. I will discuss a new public code package, DarkHistory, for exploring these possible signatures of dark matter physics in cosmological observations. DarkHistory simultaneously solves for the evolution of the ionization level and gas temperature, and for the cooling of particles produced by dark matter processes, allowing a self-consistent treatment of conventional and exotic sources of heating and ionization. I will outline some existing applications and plans for future directions.

Host: J. Lesgourgues

### Thu 07.01.2021, 16.30 h

Veronica Sanz (Valencia University)

Some examples on the use of Machine Learning techniques for LHC searches

**Abstract:**

In this talk I will present a short and pedagogical introduction to Machine Learning, and then explain some examples of its use for searches for New Physics: SMEFT effects on Higgs observables, Dark Matter searches (and their impostors), and high-momentum jets.

Host: F. Kahlhoefer

### Thu 14.01.2021, 16.30 h

Maria Petropolou (Athens University)

Magnetic reconnection in blazar jets

**Abstract:**

Blazars are a subclass of active galactic nuclei (AGN) with jets of magnetized plasma emerging from the central supermassive black hole and moving with relativistic speeds at a small angle with respect to our line of sight. Blazar emission, which is thought to originate from the jet, does not only extend over the whole electromagnetic spectrum, but also varies on timescales ranging from months to several minutes. It is still not clear whether the multi-timescale and multi-wavelength blazar variability is related to stochastic processes occurring in the accretion disk of the black hole or in the jet or what is the process that powers the observed emission. Magnetic reconnection is invoked as an efficient particle accelerator in a variety of astrophysical sources of non-thermal high-energy radiation, such as blazars. The energy dissipation and particle acceleration processes during magnetic reconnection can only be studied from first principles with fully kinetic particle-in-cell (PIC) simulations. Here, I will present the main results of our large-scale two-dimensional PIC simulations of magnetic reconnection in electron-positron and electron-ion plasmas. I will show results of radiative transfer calculations that were benchmarked with PIC simulation results and discuss their implications on the time-variable non-thermal emission of blazar jets.

Host: P. Mertsch

### Thu 21.01.2021, 16.30 h

Asher Berlin (NYU)

The Hunt for Sub-GeV Dark Matter

**Abstract:**

Recent technological and theoretical advances have reshaped dark matter science, emphasizing experimental signatures and cosmological paradigms beyond that of the canonical weakly interacting massive particle. It is now an opportune time to systematically consider alternative theories and terrestrial signatures in light of null results from recent dark matter searches. Of the various alternatives to new physics near the electroweak scale, light dark matter that is part of a larger dark sector has garnered a significant portion of the community’s interest, largely due to theoretical motivations and promising near-term prospects for detection. In this talk, I will discuss what viable and compelling scenarios exist for such light dark sectors and how they can be tested using new technology.

Host: F. Kahlhoefer

### Thu 28.01.2021, 16.30 h

Tomer Volanski (Tel Aviv)

Why Chase Ambulances?

**Abstract:**

Every now and then, a new experimental result is reported, hinting towards new physics. The subsequent "ambulance chasing” phenomena in which many papers are written shortly after the result is published, is sometimes (and justifiably) criticized. In this talk, I will discuss the positive aspects of ambulance chasing, demonstrating new ideas that stem from such studies. Two recent case studies will be considered: the EDGES sky-average 21-cm absorption signal and the XENON1T electron-recoil result.

Host: J. Lesgourgues

### Thu 04.02.2021, 9.00 h

Hiroshi Suzuki (Kyushu University)

Universal formula for the energy-momentum tensor via the gradient flow

**Abstract:**

The gradient flow is a sort of diffusion of gauge field configurations and it has been proven very useful in the context of lattice gauge theory. A salient feature of the gradient flow is that any local product (i.e., composite operator) composed of fields diffused by the gradient flow automatically becomes a renormalized finite operator. In this talk, I explain how the gradient flow enables us to write down a universal formula of the energy-momentum tensor. This formula is universal in the sense that it is independent of the regularization including the lattice regularization. I also report some application of the formula in finite temperature QCD.

Host: R. Harlander

### Thu 11.02.2021, 11.15

Andrea Thamm (Melbourne)

Flavor Probes of MeV-GeV ALPs

**Abstract:**

Axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale of the new-physics sector. Starting from the most general dimension-5 interactions, I will discuss the evolution of the ALP couplings from the new-physics scale to energies below the scale of electroweak symmetry breaking and its effects on experimental probes of ALPs with MeV - GeV masses. In particular, I will present a comprehensive analysis of quark and lepton flavor observables and derive bounds on specific ALP models in which only a restricted set of ALP couplings to SM particles occurs at tree-level.

Host: F. Kahlhoefer